Conventionally, some proposals have been made relating to a retroreflective article having an excellent wide angle performance and having improved entrance angle characteristic, observation angle characteristic, and rotation angle characteristic.
For such a cube corner retroreflective article, a lot of proposals to improve entrance angle characteristic or observation angle characteristic in particular of a triangular pyramidal cube corner retroreflective sheeting have been known since early times and various improvements have been considered. In many of these techniques, entrance angle characteristic is improved by canting an optical axis of a retroreflective element.
For example, U.S. Pat. No. 2,310,790 (Patent Document 1) by Jungersen discloses a technique in which retroreflective elements of various shapes are arranged on a thin sheet. Triangular pyramidal reflective elements exemplified in the U.S. patent include: a triangular pyramidal reflective element in which an apex is positioned at a center position of its triangular base, an optical axis is not canted, and the shape of the base is equilateral triangle; and a triangular pyramidal reflective element in which an apex is not positioned at a center position of its triangular base, and the shape of the base is isosceles triangle. It is described that with a triangular pyramidal reflective element, light can be effectively reflected to an approaching vehicle (entrance angle characteristic can be improved).
In addition, the document describes as a size of a triangular pyramidal reflective element that a depth of the element is equal to or less than 1/10 inch (2,540 μm). Further, FIG. 15 of the U.S. patent shows a pair of triangular pyramidal reflective elements having optical axes that are canted in positive (+) direction. A cant (θ) of each of the optical axes can be estimated to be about 6.5° by calculating from a ratio between lengths of a longer side and a shorter side of an isosceles-triangular base of each of the shown triangular pyramidal reflective element.
EP Patent No. 137,736B1 (Patent Document 2) by Hoopman discloses a retroreflective sheeting and a retroreflective article, in which pairs of canted triangular pyramidal cube corner retroreflective elements each having a base in a shape of isosceles triangle are arranged on a thin sheet in a rotated state by 180° with respect to each other of each pair and their bases are closely packed on a shared plane. An optical axis of the triangular pyramidal cube corner retroreflective element described in the patent is canted in negative (−) direction and it is described that the cant is about 7° to 13°.
In addition, U.S. Pat. No. 5,138,488 (Patent Document 3) by Szczech also discloses a retroreflective sheeting and a retroreflective article, in which canted triangular pyramidal cube corner retroreflective elements each having a base in a shape of isosceles triangle are arranged on a thin sheet and their bases are closely packed on a shared plane. In the U.S. patent, an optical axis of the triangular pyramidal reflective element is canted in a direction of a side shared by two triangular pyramidal reflective elements facing each other to form a pair, that is, in positive (+) direction, and the cant is about 2° to 5°. It is described that a size of the element is 25 μm to 100 μm.
In a retroreflective sheeting and the retroreflective article described in EP Patent No. 548,280B1 (Patent Document 4) corresponding to the patent, a distance between a surface including a side shared by two elements in pair and perpendicular to a shared plane and an apex of each element is not equal to a distance between a point where an optical axis of the element crosses the shared plane and the perpendicular surface, and the cant of the optical axis is about 2° to 5°. It is described that a size of the element is 25 μm to 100 μm.
In the retroreflective sheeting and the retroreflective article described in EP Patent 548,280B1 by Szczech, a cant of the optical axis is about 2° to 5° in both of positive (+) and negative (−) directions as described above. However, an example of the U.S. patent and EP Patent by Szczech described above only disclose a triangular pyramidal reflective element in which a cant of an optical axis is (−)8.2°, (−)9.2°, and (−)4.3° and which has a height (h) of 87.5 μm.
Also relating to improvement of observation angle characteristic, various proposals have been made.
U.S. Pat. No. 4,775,219 (Patent Document 5) by Appeldorn discloses a cube corner retroreflective article, in which a V shaped groove forming elements is asymmetric. An angle of the V shaped groove has a slight deviation from an angle of a theoretical V shaped groove forming a cube corner. Improvement of the observation angle characteristic is attempted by periodically changing deviation causing asymmetry of adjacent V shaped-grooves.
However, periodically changing the angle of adjacent V shaped grooves increases difficulty of die machining. Even if the difficulty can be overcome, combinations of deviations that can be provided is limited, and thus uniform spread of reflected light cannot be provided. In addition, some types of working tools such as a diamond bite for forming V shaped grooves are required for one V shaped groove direction. Further, a highly accurate working technique is required when V shaped grooves are formed asymmetrically.
U.S. Pat. No. 5,171,624 (Patent Document 6) by Walter discloses a triangular pyramidal retroreflective element, in which reflective lateral surfaces having cross-sectional shapes of a constant quadric surface are formed by using a working tool having a curved cross-section. In such a triangular pyramidal retroreflective element formed with reflective lateral surfaces having quadric surface, an appropriate divergence of retro-reflected light is possible, and thus observation angle characteristic can be improved.
However, it is of great difficulty to form a working tool having such a curved cross-section shape in an intended shape. Therefore, it has been of great difficulty to obtain the quadric surface according to the intended design due to the difficulty to work the tool. In addition, since a curved shape that can be provided is determined only by a shape of a working tool that is used, it has not been possible to form various shapes of quadric surfaces on one retroreflective article.
U.S. Pat. No. 5,565,151 (Patent Document 7) by Nilsen discloses a retroreflective sheeting, in which a part of a reflective lateral surface (A-B-H) is cut off to form a triangular prism shaped part (A-A1-A2-B2-B1-B) and a new reflective lateral surface (A2-H1-B2) that accelerate divergence of retro-reflected light so as to improve observation angle characteristic.
However, for the invention by Nilsen, specific description about a preferable shape of triangular prism to be provided and a preferable angle with which new reflective lateral surfaces are formed is not much provided. In addition, a special tool for cutting off a part of a reflective lateral surfaces to form a triangular prism shaped part is required. Further, the newly formed triangular prism shaped element does not have a function of retroreflection but is intended to attain spread of retro-reflected light by merely dispersing light in various directions.
However, the technique for improving entrance angle characteristic and observation angle characteristic cannot improve rotation angle characteristic.
Also relating to improvement of rotation angle characteristic, various proposals have been attempted, but any of the proposals intends to improve rotation angle characteristic by combining directions of retroreflective elements having triangular shaped bases divided into various regions.
As documents describing such improvement, U.S. Pat. No. 5,022,739 (Patent Document 8), U.S. Pat. No. 5,132,841 (Patent Document 9), and U.S. Pat. No. 5,175,645 (Patent Document 10) by Bennett et al., U.S. Pat. No. 6,036,322 (Patent Document 11) by Nilsen, U.S. Pat. No. 5,706,132 (Patent Document 12), and U.S. Pat. No. 5,936,770 (Patent Document 13) by Nestegard, and U.S. Pat. No. 5,898,523 (Patent Document 14) by Smith can be exemplified, for example.    [Patent Document 1] U.S. Pat. No. 2,310,790    [Patent Document 2] EP Patent No. 137736B1    [Patent Document 3] U.S. Pat. No. 5,138,488    [Patent Document 4] EP Patent No. 548280B1    [Patent Document 5] U.S. Pat. No. 4,775,219    [Patent Document 6] U.S. Pat. No. 5,171,624    [Patent Document 7] U.S. Pat. No. 5,565,151    [Patent Document 8] U.S. Pat. No. 5,022,739    [Patent Document 9] U.S. Pat. No. 5,132,841    [Patent Document 10] U.S. Pat. No. 5,175,645    [Patent Document 11] U.S. Pat. No. 6,036,322    [Patent Document 12] U.S. Pat. No. 5,706,132    [Patent Document 13] U.S. Pat. No. 5,936,770    [Patent Document 14] U.S. Pat. No. 5,898,523